To assist us in selecting the most appropriate method of treatment, additional studies may be required on an outpatient basis. For the studies noted below, our office will gladly assist you with scheduling.
Click on the headings below to learn more about each additional study.
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) is a very useful study, but it is often overused as well. It is well known that there are many false positives and false negatives. Often physicians who are not orthopedic surgeons order expensive ($1500) scans when a simple x-ray would tell an orthopedic surgeon what the problem is. Most general physicians are not used to interpreting x-rays on their own. In this instance the MRI is just wasted money. In my opinion, musculoskeletal MRI s should only be ordered by orthopedic surgeons after they have evaluated the patient (cost $300). This would result in large health care savings.
Many people believe that you cannot have an MRI if you have implanted metal. This is not true. Most implanted metals are not ferromagnetic and are not affected by the MRI magnets. The exceptions are metal fragments that get into the body accidentally, including shrapnel as well as cardiac pacemakers. Hopefully MRI compatible pacemakers will soon become available.
If an MRI is taken around an orthopedic implant there is no danger to the patient. The metals we use: stainless steel, cobalt chrome, and titanium are not magnetic. The problem with scanning around an implant is that artifact is created around the implant on the diagnostic image. This artifact makes the image difficult to interpret. New software to eliminate this artifact have been developed and are called metal suppression or metal artifact reduction sequences (MARS). Not all radiologists are familiar with these, and useless scans around implants are still occasionally performed. The radiologists that I use are well versed in these techniques, but sometimes patients who don’t live here need to get scans done near their home. I suggest finding a larger center that employs a musculoskeletal radiology subspecialist who is likely to perform the scan correctly.
In most cases of moderate to severe arthritis an x-ray is all that is needed to make the diagnosis. Hip dysplasia, various impingement deformities, and stage 3 and 4 necrosis are also best evaluated by simple x-rays. A hip MRI is a useful study to look for problems such as stage 1 and 2 osteonecrosis, hip abductor tears, and labral tears. An arthrogram MRI (dye is injected into the joint prior to the MRI) is better when we look for a labral tear.
MARS MRI and Metal Suppression CT
MARS MRI and metal suppression CT are useful for diagnosing metal adverse wear failures (AWF) if metal ion testing shows excessive elevation.
I also use MRI prior to knee replacement surgery to create 3 dimensional virtual models of the operative leg and plan the knee replacement surgery. The company that provides this service creates sterile custom guides according to my protocol that are used in surgery to improve the accuracy of the bone cuts resulting in more reproducible alignment of the implants.
Finally MRI are very useful to evaluate the lumbar spine for arthritis, herniated discs, and stenosis. These conditions may cause referred pain to the hip area confusing both the patient and the orthopedic surgeon. This is particularly useful after hip replacement if there is residual buttock, lateral hip or sciatic type pain.
CT scans are also very useful to evaluate for adverse wear failure (AWF) around metal bearings in certain circumstances. Similar to simple x-rays, but unlike MRI they do expose the patient to a low dose of x-rays. They are less costly and allow imaging of both hips simultaneously. Metal suppression techniques also need to be used.
We are currently working on advanced techniques to accurately measure implant positions in 3 dimensions on these scans. We hope this will eventually allow us to develop more accurate guidelines as well as surgical guides for implantation during surgery.
DEXA scans (dual energy x-ray absorptiometry) is the test we use to measure bone density. The result is reported as a T-score, which is a way to compare a patient’s bone density to a normal young population. Because bone density varies according to race and gender, different normal populations are used. A T score between 1 and -1 indicates that your bone density is within the range of 90% of the 26 year old normal population of your reported race and gender.
If you Tscore is below -1.5 you are said to have osteopenia. If it is below -2.5 you have osteoporosis. This means weak bone. These cutoff levels are arbitrary. The lower your bone density, the higher your fracture risk. Osteoporosis, unlike osteoarthritis, is not painful. Pain occurs when a bone breaks. Bone density gradually decreases for the entire population with age. Women can have a more rapid rate of decline in their bone density for the first 5 years after onset of menopause. All women should have a baseline DEXA scan when they start menopause and this should be repeated at least once 2 years later.
To better understand osteoporosis I will use the analogy of high blood pressure. Neither condition is typically symptomatic. Left untreated, these conditions will eventually lead to severe medical problems; heart attack and stroke on the one hand, and fractures on the other. Both conditions can be treated to prevent later complications. Numerous medications are now available to treat low bone density. There is not complete agreement at which point treatment should be recommended.
I obtain DEXA scans on every patient on whom I perform a joint replacement operation. The cost is low ($150). We have found that this test is very predictive of certain complications that can develop after surgery. Sometimes even very young patients can have low bone density. In hip resurfacing we use the result in determining the risk of postoperative femoral neck fracture and determining how to precisely prepare the socket in surgery. Patients who have weaker bone are treated with anti resorptive medication and a longer period on crutches after the surgery. We have reduced the rate of the complication of femoral neck fracture from 1-2% to 0.1% using this strategy.
Metal Ion Level Testing
Metal Ion Level testing is a valuable way to screen for excessive wear of metal bearing implants. We recommend routinely measuring cobalt chromium and titanium levels 2 years after surgery on every patient in order to discover adverse wear failures early. In certain patients we recommend repeat monitoring thereafter. Most patients with metal bearing hip replacements as well as metal-plastic bearing knee replacements have some elevation of cobalt and chromium in their blood. There is no known problem caused by this.
Speculations of increased cancer risk have been disproven by numerous long-term studies. There is no evidence that kidney damage can occur due to circulating ions, but abnormal kidney function can lead to ion level elevation in the blood. There is no agreed upon acceptable level for joint implant patients. The normal levels in patients without implants is about 2 ug/L. We have done extensive studies on metal ion levels. The lowest level we have seen associated with an adverse wear failure (AWF) is 15ug/L. Sometimes a level higher than this is seen without evidence for AWF.
Implant loosening may be another failure mode that can be associated with elevated levels. There have been a handful of case reports of generalized systemic side effects possibly caused by very high metal ion levels. We have seen more than 10 AWF cases with levels between 50-200ug/L and none exhibited any systemic symptoms. It appears that systemic effects do not occur at levels that are typically seen in well functioning cases, and are even rare with very high levels seen in AWF cases. We obtain levels at 2 years because it is known that implants go through a run-in wear period of 1-2 years. Less is known about what is an acceptable level during this time. The earliest time that we have ever seen an AWF is after 2 years; it takes time to develop these problems.
We currently recommend metal ion testing:
- Routinely on all patients at 2 years postop
- When evaluating a painful hip implant
- Prior to performing a second hip surgery on the opposite hip
- Repeat levels periodically if levels are above 10ug/L, there is a steep acetabular inclination angle, the patient has a recalled DePuy ASR implant
We recommend obtaining a metal suppression CT or MARS MRI whenever we encounter a ion level above 10ug/L. Significant soft tissue inflammatory mass (not cancer) or large fluid collections indicate a probable AWF. All case of AWF so far have occurred in cases where the acetabular inclination angle is above 50. But only 5% of patients with AIA>50 develop AWF.
Bone and Indium Scans
Bone and Indium scans are performed by removing some of your own blood and labeling it with an isotopic material that is then re-injected. You will return one day later and the area of the joint will be scanned. These tests help us evaluate for loosening of implants or for infection. The amount of radiation is generally not much more than a single x-ray exposure. Complications have been minimal. Computerized scanning is sometimes combined with injection of these agents to better define the changes in a three-dimensional way.
Aspiration & Arthrogram
Aspiration & Arthrogram is typically done to obtain information about the presence or absence of infection, particularly if you have had previous surgery. The radiologist may be able to aspirate fluid directly from the hip joint, which will provide valuable information. These samples can be analyzed by cell counts culture gram stain and PCR (a DNA test for bacteria).
Final culture results are usually available in 7 - 10 days. In general, these procedures are not very painful; local anesthesia is utilized in the skin. On occasion, mild discomfort may be associated with the study. Most often it is transitory and usually can be relieved by medications.
Blood tests such as CBC, CRP, and ESR are sometimes done as screening tests to rule out infection. If they are positive we usually follow this up with either an indium scan and/or aspiration.